1. Field of the Invention
The present invention is directed generally toward pseudomorphic and dislocation free heteroepitaxial structures, and methods of forming the same. In particular, the subject invention relates to such structures in which pseudomorphic and heteroepitaxial layers are grown on thin, free-standing substrates.
2. Description of the Prior Art
In the semiconductor industries, many devices, such as lasers and transistors, of very high performance and of peculiar characteristics are made of a class of materials called heteroepitaxial or pseudomorphic structures. A heteroepitaxial structure is one in which a semiconductor is epitaxially grown on a substrate of different lattice constant. If the heteroepitaxial layer is thinner than a certain dimension, the so called "critical thickness," the elastic energy in the system is not large enough to create misfit dislocations, and a pseudomorphic structure is produced. Such dislocations, if produced, would detrimentally effect device performance.
Since the introduction of pseudomorphic epitaxy, this structure has been widely applied to high performance electronic and optoelectronic devices. Because of the extra degree of freedom offered by the pseudomorphic structure, researchers can do ingenious bandgap engineering to not only improve existing device performance but also to create new devices. For all the high quality pseudomorphic structures so far reported, the pseudomorphic layer thickness is thinner than the critical thickness, thus preventing the formation of misfit dislocations. For example, in devices such as quantum well lasers, the strained active layers are made thinner than the critical thickness. However, for many other applications, such as detectors and modulators, pseudomorphic layers much thicker than the critical thickness are desirable.
Another critical problem for many semiconductor heteroepitaxial devices is the density of threading dislocations. The existence of threading dislocations is detrimental for many devices, such as lasers and photodetectors. In conventional growth conditions, once the misfit dislocations are developed, they are driven towards the layer surface by an image force, resulting in an undesirable situation.
It is clear from the foregoing that there exists a need for pseudomorphic and heteroepitaxial structures which can be grown or formed of any arbitrary thickness without the formation of misfit or threading dislocations, regardless of the critical thickness of the structure.